Fate of the organophosphorus insecticide dichlorvos in stored beans

Under local conditions, 14C-labelled dichlorvos was applied at 12 and 24 mg insecticide/kg seeds by spraying faba and soybeans before storage. The amount of external, internal and grain-bound insecticide residue was estimated at intervals during 30 weeks of storage. External residues were found to decrease with time to reach about one-fifth of the initially applied dose on both bean species by the end of the storage period. The rate of penetration of the insecticide was higher in soybeans than in faba beans and was apparently dose dependent in soybeans. The bound residues increased with time of storage and the maximum binding amounted to 8–10% and 9–11% of the actual dose applied in faba beans and soybeans, respectively. The incomplete recovery of the actual radioactivity by extraction and combustion may be attributed at least in part to volatilization of the parent substance. Chromatographic analysis of external extract consisted mainly of unchanged insecticide. The internal extract of grains revealed the presence of dichlorvos and desmethyl dichlorvos, which were present in free and conjugated forms. Dimethylphosphate and monomethylphosphate were identified as hydrophilic 14C-degradation products.